In optical record carriers information is encoded in a pattern of optically detectable marks and of spaces between the marks in an information layer of the optical record carrier. These marks may be in the form of embossed pits, such as for example in Read Only type record carriers, in the form of changed optical properties of a dye layer, such as for example in Recordable type record carriers, or in the form of amorphous areas in a crystalline layer, such as for example in Rewritable type media. In general these marks are stored along a spiral shaped or concentric shaped track on the information layer of a disc like optical record carrier. An optical record carrier may comprise a single information layer or multiple information layers separated by spacer layers, such as for example a Dual Layer disc consisting of two information layers.
In a reading device information is retrieved from such an optical record carrier by irradiating the information layer of the optical record carrier by a radiation beam, such as for example a laser light beam, and detecting the radiation beam reflected from the optical record carrier. When the radiation beam scans the information layer along the track, the reflected radiation beam is modulated according to the pattern of marks and spaces stored on the information layer. This modulated reflected radiation beam is transferred into a modulated electrical signal by a detector. By decoding this modulated electrical signal the information stored on the optical record carrier is retrieved.
The Signal-to-Noise Ratio (SNR) of the modulated electrical signal representing the information read from the optical record carrier depends on the optics and electronics of the reading device. In general the SNR of the modulated electrical signal decreases with increasing reading speed. This because at increasing reading speeds the bandwidth of the detection electronics in the reading device needs to increase, thereby increasing all kinds of noise contributions, such as noise of the radiation source, electronic noise, shot noise, etcetera.
When reading optical record carriers, a more optimum SNR is obtained in case of high radiation levels. The signal level of the modulated reflected radiation beam, and thereby the signal level of the modulated electrical signal, is proportional toPr·M·R,where Pr is the power of the radiation beam (often referred to as the read power), M is the modulation of the marks and spaces, and R is the reflectivity of the information layer.
It is a problem of current optical systems, especially when using optical record carriers having a low reflectivity, such as for example Dual Layer DVD Rewritable systems and Recordable and Rewritable BluRay Disc systems, that the Signal-to-Noise Ratio of the modulated electrical signal representing the information read from the optical record carrier, often referred to as the read-out signal, becomes too low at higher read-out speeds when using conventional read powers, thus causing errors in the retrieved information. This is especially the case in low cost systems, where, in general, the efficiency of the optical path from the record carrier to the detector is low.
In future optical systems where the number of information layers in the optical record carrier is increased to three or more, the Signal-to-Noise Ratio of the read-out signal may even further decrease due to the decreasing reflectivity of the individual information layers with increasing number of stacked information layers in a record carrier.